mm/filemap: don't call folio_test_locked() without a reference in next_uptodate_folio()

The folio can get freed + buddy-merged + reallocated in the meantime,
resulting in us calling folio_test_locked() possibly on a tail page.

This makes const_folio_flags VM_BUG_ON_PGFLAGS() when stumbling over the
tail page.

Could this result in other issues?  Doesn't look like it.  False positives
and false negatives don't really matter, because this folio would get
skipped either way when detecting that they have been reallocated in the
meantime.

Fix it by performing the folio_test_locked() checked after grabbing a
reference.  If this ever becomes a real problem, we could add a special
helper that racily checks if the bit is set even on tail pages ...  but
let's hope that's not required so we can just handle it cleaner: work on
the folio after we hold a reference.

Do we really need the folio_test_locked() check if we are going to trylock
briefly after?  Well, we can at least avoid a xas_reload().

It's a bit unclear which exact change introduced that issue.  Likely, ever
since we made PG_locked obey to the PF_NO_TAIL policy it could have been
triggered in some way.

Link: https://lkml.kernel.org/r/20241129125303.4033164-1-david@redhat.com
Fixes: 48c935ad88 ("page-flags: define PG_locked behavior on compound pages")
Signed-off-by: David Hildenbrand <david@redhat.com>
Reported-by: syzbot+9f9a7f73fb079b2387a6@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/lkml/674184c9.050a0220.1cc393.0001.GAE@google.com/
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: "Matthew Wilcox (Oracle)" <willy@infradead.org>
Cc: Hillf Danton <hdanton@sina.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/filemap.c

@@ -3501,10 +3501,10 @@ static struct folio *next_uptodate_folio(struct xa_state *xas,
 			continue;
 		if (xa_is_value(folio))
 			continue;
-		if (folio_test_locked(folio))
-			continue;
 		if (!folio_try_get(folio))
 			continue;
+		if (folio_test_locked(folio))
+			goto skip;
 		/* Has the page moved or been split? */
 		if (unlikely(folio != xas_reload(xas)))
 			goto skip;